Published: 30 April 2020 | Category: Technical advice note | Audiences: Road traffic engineers & consultants, Roading contractors
This note is to advise that there is an issue with the use of GAP65 as a drainage material and, as a result, this material must not be used unless its suitability for the works as designed is demonstrated. The new requirement applies from the date of this notice.
This note is to advise that there is an issue with the use of GAP65 as a drainage material and, as a result, this material must not be used unless its suitability for the works as designed is demonstrated. The new requirement applies from the date of this notice.
Failure to achieve appropriate drainage both behind retaining walls and within geotechnical structures will result in increased hydrostatic pressure which will not have been allowed for within the design and the potential for premature failure of the structure before the end of the design working life. The alternative is to provide a structural design to allow for hydrostatic pressure, which will result is larger structural members and additional associated works. This will result in significant cost increases which would reduce available funding for further projects. The outcome is considered unreasonable, given the limited costs associated with providing a proper drainage medium at the time of construction.
The processes and procedures outlined in this update must be followed by Waka Kotahi the NZ Transport Agency for all structure drainage works undertaken from now on.
Waka Kotahi must ensure that its practices deliver proper value and do not cause medium-term issues that are likely to increase costs. In this case, the use of GAP65 as a drainage medium without proper consideration of its suitability to perform the role has had some unacceptable consequences.
The purpose of the notice is to advise that there has been an issue and to ensure action to obviate further instances. It also allows our suppliers to be more aware of our expectations for the importance of suitable design of drainage features.
There is an increasing use of GAP65, or a ‘modified’ GAP65 grading, as a drainage medium behind and within retaining and geotechnical structures. Adoption of such materials does not provide appropriate drainage for the design working life of such structures, but rather a lesser period that varies with the grading of the material. GAP65 is therefore not acceptable as a drainage material unless permeability tests undertaken on the intended material can demonstrate suitability for the full design working life of the structure. Such tests need to be on the material proposed and not quarry approval tests.
Specific approval, through the departure process, will be required to use GAP65, or a grading-modified GAP65, as a drainage medium. The departure request will require sufficient evidence to demonstrate significant benefits for Waka Kotahi at reasonable cost and acceptable risk, which may include maintenance regimes to ensure the drainage function is maintained.
Our target audience for this advisory note is generally internal staff giving approvals and those external agents designing road structures on behalf of Waka Kotahi. As a result, our communications will mostly be focussed internally. Advice of these changes will be promulgated by:
The implementation should be fairly simple as the audience is specific and small. The aim is to target that audience using the communication exercise.
Given this update is a single, specific issue, it was not considered necessary to conduct company-wide, specific training on the changes.
It is important to check that this this has been delivered effectively and the desired outcome achieved. A review of the effectiveness of the changes will be undertaken by sampling the designs used for a number of structures, particularly walls, used in road construction projects. Should the advice not be being heeded, a larger sample will be taken to establish the problem size and a further communication exercise will be undertaken.
Author: Waka Kotahi New Zealand Transport Agency
Published: March 2020
Version: 1
Found at: https://www.nzta.govt.nz/roads-and-rail/highways-information-portal/technical-advice-note/
If you have any questions or feedback, please contact the Lead Advisor - Geotechnics at geotechnics@nzta.govt.nz
[signed]
David Darwin - Chair, Ratification Group
Vanessa Browne - Delegated Approver
